1. Field of the Invention
The present invention relates to a power amplifier which is to be connected and powered by a DC battery, and which is provided with circuitry for preventing destructive breakdown that may be caused by inadvertently connecting the DC supply to the output terminal of the amplifier while the amplifier is unpowered.
2. Discussion of the Related Art
During the installation of devices having power amplifiers, destructive breakdown events caused by inadvertent erroneous connection of the device to a local DC supply are frequent. The most typical instance is in the installation of audio amplifiers, radio receivers, and cassette and CD players on vehicles.
The output terminals of power amplifiers are normally provided with some sort of protection against accidental shortcircuits that may inadvertently occur to ground potential or to the DC supply voltage. However, there are still instances of failures on the field for which specific protection has not yet been devised.
Among these remaining accidental causes of failure there is a condition commonly known as "short-to-plus-unpowered" (SPU). Such a condition, which may occur during the installation of an apparatus on a vehicle, is schematically depicted in FIG. 1. An SPU error is caused by an inadvertent connection of the output terminal (Out) of the amplifier (Amp), to the positive pole of the battery (V.sub.batt) of the vehicle, instead of correctly connecting the supply terminal (Vcc) to the positive pole of the battery (V.sub.batt). This error may occur when the isolated cable coming from the positive pole of the vehicle's battery is erroneously connected to the output terminal of the card that contains the amplifier. In practice, the output terminal (Out) of the amplifier becomes connected to ground through a filter capacitor C1, typically about a thousands microfarads, that is typically and through an equivalent resistance, typically in the order of 10 ohms, toward ground R1 of the rest of the circuit on the card.
When the output terminal (Out) of the circuit is accidentally connected to the positive node (V.sub.batt) of the local battery (e.g. of the vehicle), the circuit is "powered" through its output node. Current flows toward ground both through the ground terminal (Gnd) of the amplifier, as well as through the supply terminal (Vcc) and the equivalent network formed by R1 and C1, the latter being charged by the current flowing through the Vcc terminal of the amplifier.
Commonly, the output stage of a power amplifier has a push-pull configuration and includes a pair of complementary transistors, such as an NPN and a PNP transistor, having their respective collectors connected to the output node of the amplifier (Out). An equivalent circuit, under an SPU condition, is depicted in FIG. 2. In other instances, the output stage of the amplifier may have a bridge configuration and may be formed by transistors of the same type.
In all cases, power transistor Q1 is functionally connected between the supply rail and the output node of the stage, and, as depicted in the figure, may include a PNP transistor, such as a vertical, isolated collector, PNP transistor (ICVPNP) having an emitter connected to the supply line Vcc of the amplifier. Transistor Q1 functions in a reverse bias active region, where the collector terminal acts as the emitter (E), the base terminal (B) as a base and the emitter terminal as a collector (C).
As in any transistors, also in a reverse bias configuration, the voltage between the "base" and the "collector" should not become greater than a maximum value (dependant on the fabrication technology) in order to avoid breakdown of the junction with an attendant risk of a destructive breakdown because of an excessive power dissipation.
For many fabrication processes, the maximum voltage that can be tolerated between emitter (E) and collector nodes, under reverse bias conditions, is less than the battery voltage (V.sub.batt). This makes an SPU condition particularly critical, especially immediately after the occurrence of an accidental short circuit, when the capacitor C1 is discharged and the voltage between the "base" and the "collector" assumes a maximum value that is equal to about V.sub.batt -0.7 V.
It is evident that there is a need and for a device capable of preventing failure of the transistor Q1 under SPU conditions. No solutions to this problem are currently known.